If someone shoots a gun (45 caliber) at someone holding up a table or desk 10 feet away, does it slow or turn the bullet away? Assume it's a standard .45 caliber handgun. Shooter is 10 feet away, and we hold a desk, mini-table, blackboard, etc on our chests(to protect vitals). 
How do you figure out the bullet's decrease in velocity, and likelihood of flying away from you. 
I got the idea thinking  whether could we protect ourselves against a madman like the VT or Columbine guys?
 A: In order to answer this question we must make some more assumptions.  Given your scenario, one of the tragic school shootings which occurred in the States, we can assume the shooter will be within 50 feet.   Since we are at a school, then let us assume typical materials found there such as desks and tables composed of laminate or other glued wooden materials augmented by plastic.  With these assumptions, any bullet from a .22 caliber (~ 2 oz) and larger at close range will most likely penetrate the barrier.  
When a bullet impacts upon a surface of a material as given above, it penetrates, begins to deform and may fragment in the material. To determine how it slows and deforms, we must consider the size, shape, velocity and composition of the bullet, the interaction of the bullet with atmosphere, such as its dynamic stability, as well as the material response of the material being hit.  Laminates should exhibit an inelastic response to the bullet adding to the difficulty of a priori calculating what will happen besides simply saying that it will go through or not. 
Most bullets are made of lead surrounded (jacketed fully or partially) by a harder metal such as copper.  This is done as a trade off between ballistic stability and deformability in order to wound. There are several types of bullets, the 2 most common are full metal jacket bullets, which are conical at the tip and various hollow tip styles which are blunt tipped and designed to expand as they penetrate the material since the segmented jacket peels back at it travels through the material creating a larger wound channel due to the high pressure zone commonly associated with hollow-tip bullets. 
A  bullets decrease in velocity due to traveling through a material is a nontrivial calculation which is usually not even attempted.  Typically, the velocity given is an average muzzle velocity and the energy in foot pounds is given as the total  amount of energy dumped into a ballistic pendulum, at a given distance, exhibiting an elastic impulsive response to the collision.  The bullet hits it, the pendulum moves, and knowing facts about the bullet and pendulum, we calculate the energy deposited in the collision as an average. 
Ballistics is an experimental science, in which bullets are created according to various known criteria and then tested against various barriers while measuring performance before and after penetration.  For handguns, after a certain point, bigger is not always better and jacketed blunt bullets (hollow tips) can cause a more lethal wound than a full metal jacket despite the fact that they may not penetrate as far in ballistic gelatin. It is possible that bullets of different sizes but traveling at varying speeds will give similar wound profiles.
A .45 caliber bullet is capable of penetrating a typical hollow cinderblock if it is jacketed.  Given what you present, desks or tables provide little cover, but better than none at all. Real examples of ballistics tests can be found here:
http://www.ballisticsbytheinch.com/
http://www.theboxotruth.com/docs/bot1.htm
